1. Field of the Invention
This invention relates generally to methods for forming at least one electrodeposited coating over a coated substrate, such as forming an electrodeposited coating over a glass substrate having at least one conductive coating, and to articles made thereby.
2. Description of the Currently Available Technology
“Electrodeposition” or “electrocoating” processes are used in a variety of manufacturing fields. In a typical electrocoating process, a metal substrate is immersed in a bath containing an electrocoating composition. The metal substrate serves as a charged electrode in an electrical circuit defined by the electrically charged metal substrate and an oppositely charged counter-electrode. Sufficient current is applied between the electrodes to deposit a substantially continuous, adherent film (electrocoat) of the electrocoating composition onto the surface of the metal substrate.
Electrodeposition has become the primary method for applying corrosion-resistant primers onto metal automotive parts. Additionally, in the field of printed circuit boards, an electrodeposited coating can be applied onto a metal “core” and then portions of the electrodeposited coating ablated in a predetermined pattern to expose sections of the conductive metal core to form electrical circuits. Examples of some known electrodeposition processes are disclosed in U.S. Pat. Nos. 4,333,807 and 4,259,163.
In known electrodeposition processes, the applied electrocoat is typically opaque to hide the underlying substrate. Moreover, the substrate upon which the electrocoat is electrodeposited is typically a solid metal part, such as an automotive or appliance component. Metal parts are well suited to the electrodeposition process since they can be relatively easily charged to function as an electrode in the electrodeposition process.
In a relatively recent development in the automotive industry, organic primer compositions containing metal particles have been developed to provide the underlying metal automotive component with increased corrosion protection. For example, U.S. Pat. No. 4,346,143 describes a zinc-rich organic primer applied over a ferrous metal substrate to provide corrosion protection. The organic primer contains zinc particles or zinc dust, color pigments, and a resinous binder. Since the pigment and zinc particle-containing resinous primer is electroconductive, the primer can be subsequently topcoated using an electrocoating process. U.S. Pat. No. 6,008,462 discloses a weldable resinous coating composition having a resin, a crosslinker, and conductive iron powder particles randomly dispersed in the composition. In these known conductive organic coatings, the metal particles are randomly distributed throughout the organic coating material and the coating is typically applied to a sufficient thickness to hide the underlying metal component and/or to provide corrosion protection for the underlying metal part.
It would be advantageous to utilize the electrocoating process in other coating environments, such as to coat non-conductive substrates, such as glass, ceramic, and tile, just to name a few. However, utilizing non-metal substrates in an electrocoating process presents several problems. For example, electrodeposition requires the ability to electrically charge the substrate to be coated to act as an electrode during the electrocoating process. This is not possible with a non-conductive substrate, such as glass. While conductive organic coatings such as those described above might be applied to a glass substrate to provide an electroconductive surface, such resinous primers could adversely limit the end uses of the resultant coated glass piece. For example, glass panes having a functional coating, such as a solar control coating or an aesthetic coating, are used in automotive and architectural applications. These coated glass panes are typically required to have predefined optical and solar control properties, such as a minimum visible light transmittance, solar infrared reflectance, reflected color, and the like. The presence of a pigmented resinous primer could adversely impact upon the desired optical and/or solar control properties of the coated glass. Additionally, at the elevated temperatures commonly used to coat glass sheets, such metal particle-containing resinous primers could decompose or disintegrate to the point where they would no longer provide a conductive surface suitable for electrodeposition.
Therefore, it would be advantageous to provide a method for electrocoating a substrate, such as but not limited to a glass substrate having a conductive coating, that reduces or eliminates at least some of the drawbacks described above.